As is well known, synchronous rectifiers are known to be used in electric power converters. When the power converter incorporates a transformer for galvanic isolation, the control of the synchronous rectifiers becomes critical to the proper operation of the power converter. A synchronous rectifier, as used in a power converter employing a transformer, can be defined as a circuit employing two or more synchronized field effect transistors (SynchFETs) or metal oxide field effect transistors (MOSFETs) as rectifying devices and related drive circuitry to control the on-off cycling of the MOSFETs. A synchronous rectifier is a switch that is controlled to behave as a rectifier does such that the switch is in the ON-state when the rectifier would be conducting current and in the OFF-state when the rectifier would be blocking voltage. The switch can be chosen to have a lower voltage drop when conducting current reducing the power loss and increasing the efficiency. MOSFETs are a good device to choose to use as a synchronous rectifier because they contain an intrinsic diode that allows them to behave as a normal rectifier. This reduces the timing requirements for the control circuit. In addition, state-of-the-art devices are now available with very low ON-resistance that greatly decreases the power loss for high current converters. Those skilled in this art call a MOSFET that is used as a synchronous rectifier a SynchFET.
The control of the synchronous rectifier determines the improvement the synchronous rectifiers give to the efficiency of the power converter. As is well known, several methods historically have included deriving the control function from the waveforms created by the transformer and deriving the control function from the control of the main converter switches on the other side of the transformer, the primary side. Such arrangements do not take full advantage of the synchronous rectifier to conduct the power at maximum possible duty-cycle.
As is also well known, existing synchronous rectifier circuits result in a "utilization rate" of the SynchFET's (the percentage of the time that they are in the ON-state) of 50% or less. This percentage will vary with the input voltage and output current to satisfy the regulation control parameters.
What is needed is a synchronous rectifier control method and power topology that addresses these and other problems in a simple, efficient, cost effective manner.